1. Field of the Invention
The present invention relates to automatic gain control systems wherein a signal of variable average amplitude is demodulated and the demodulated signal is used to generate a control voltage for regulating the gain of a prior amplifier in a sense to hold the demodulated output at a nearly constant average amplitude.
The automatic gain control system herein provided more specifically relates to television signal reception, wherein a remotely transmitted signal contains pulses of constant modulation index which may be used to define the proper signal level.
2. Description of the Prior Art
Automatic gain control systems are commonplace in receivers for the reception of remotely transmitted signals. This is true in standard amplitude modulation receivers and in television reception systems used in the home, as well as in commercial receivers for other classes of remote transmission. In all such systems, the received signal is sensed to obtain a measure of its received amplitude, and then an electrical quantity proportional to that amplitude is used to regulate the amount of amplification performed on the signal. In the case of television transmission, the signal contains synchronizing pulses which bear a fixed relation to the signal level, representing the maximum carrier level. Thus, if one separates the pulses from the video portion of the signal, one may generated a d.c. gain control voltage which has an average value proportional to the pulse amplitude. This voltage may then be used to regulate the gain of the tuner and the intermediate frequency amplifier of the television receiver to hold the signal at a constant level at the video demodulator.
In the conventional television receiver, the separation of the pulses from the video signal occurs by using a timing pulse created during horizontal "fly back" in the cathode ray tube deflection circuit. The timing pulse is then used to gate on an amplifier stage during the pulse period to separate the pulses from the video portions of the signal. The gated amplifier produces a succession of output pulses, which are applied to a filter to develop a smoothed d.c. voltage, which becomes the gain control voltage. The automatic gain control circuit normally requires appreciable amplification, since the pulse duty cycle is only about 10%.
If not externally gated, the amplification means recharges the smoothing filter through a diode. The recharging rate is then made fast enough to avoid the customary visual flicker problems for the rising part of the signal level. However, the falling part of the signal level depends upon the natural time constants of the RC network to discharge the filter. Since the charging duty cycle is short and droop between the pulses undesirable, the natural time constants of the filter tend to be too long to follow rapid decreases in signal strength. The result is that the overall amplitude regulation of the signal is less than optimum when fast response is desired. AGC circuits without external gating are normally not used in TV receivers because of inability to meet the combined requirements of droop, freedom from aircraft flutter and noise immunity.
Some systems have been developed which provide for fast attack and fast release of the AGC voltage. These, however, have been for applications which are intended to have no d.c. modulation, i.e., in conventional AM receivers. The time constants of such a system are then chosen to average the modulation effects to zero before any action is taken to change the gain of the system relative to the average carrier level. These systems are not applicable to a NTSC TV signal which contains arbitrary amounts of d.c. modulation as a function of picture content.